This invention relates to satellite communications systems using multiple spot beams from a geosynchronous earth orbit satellite to provide selective coverage of the continental United States and, more particularly, relates to a system having a satellite receiving hub in every spot beam that allows for asynchronous communications between each hub and the satellite for maximizing frequency re-use.
The rapid growth of the Internet and the unavailability of high-speed connections from standard telephone lines and local cable providers have resulted in an intense search for an alternative high-speed mode of communications. Satellite communications (xe2x80x9cSATCOMxe2x80x9d) systems are a natural selection for replacing conventional land-based communications systems as a means of providing high-speed digital communications new links.
Typical SATCOM system configurations are shown in FIGS. 1-3. FIG. 1 is an illustration of a SATCOM xe2x80x9cbent-pipexe2x80x9d configuration for two ground terminals located within the same beam. In the bent-pipe configuration, a first ground terminal 102 transmits a signal on the uplink frequency band to a GEO satellite 108. Upon reception of the signal, the GEO satellite shifts the frequency of the signal to a downlink frequency and retransmits the signal to the second ground terminal 104. The xe2x80x9cbent-pipexe2x80x9d configuration does not require the satellite to have on-board processing. Rather, the satellite merely acts as a relay from one ground terminal to another ground terminal. Because the satellite does not have on-board processing, the xe2x80x9cbent-pipexe2x80x9d configuration is typically limited to use within a single beam 106.
Another standard SATCOM configuration is shown in FIG. 2, which illustrates a SATCOM xe2x80x9chubxe2x80x9d configuration. In the xe2x80x9chubxe2x80x9d configuration, a series of ground terminals 202, 204 and a single hub 206 are located within a single beam 208. The hub acts as a two-step bent-pipe configuration, in which the uplink signal is routed from the GEO satellite 210 to an intermediate ground hub 206. The hub acts as a local control center to assign channels and other functions associated with the network management. The intermediate stop typically adds an additional xc2xc-second to the signal propagation delay normally associated with the round-trip to a GEO satellite, which is unacceptable for high-quality telephony services. In order to avoid this additional delay, the hub configuration can also operate as a xe2x80x9cbent-pipexe2x80x9d configuration, in which the hub is bypassed and the downlink signal is routed directly to a second ground terminal.
Additionally, ground terminals within the hub configuration may also operate in a one-way xe2x80x9cbroadcastxe2x80x9d mode, in which a single ground terminal transmits an uplink signal to the GEO satellite, which shifts the frequency for transmission on the downlink channel. However, instead of simply transmitting the downlink signal to a single ground terminal, the satellite xe2x80x9cbroadcastsxe2x80x9d the signal over the downlink channel to every ground terminal within the beam.
Still another standard SATCOM configuration is shown in FIG. 3, which is an illustration of the SES ARCS SATCOM system. The ARCS SATCOM system combines DVB technology on the downlink signal with a high-speed satellite uplink signal. The ARCS SATCOM system uses a standard Ku-Band DVB downlink 314 and a xe2x80x9cpiggybackxe2x80x9d Ka-band payload, which routes Ka-band uplinks 316 from individual ground terminals 304 to a single hub 306, located in Luxembourg. The ARCS SATCOM system provides eight beams 302 on the Ka-band uplink, each of which has a footprint of approximately 500 miles diameter on the earth. As a result of this high gain from the receive antenna on the satellite 308, a dish only 75 cm in diameter with a xc2xdW transmitter can provide 144 Kbps return channel data rate. The Ka-band uplinks from all eight of the beams are returned to the single hub in Luxembourg for processing. The DVB video data for the Ku band is broadcast on an uplink signal 312 to the satellite from the hub 306, and is re-broadcast on the downlink signal using Ku-band DVB transponders.
Conventional SATCOM systems using geosynchronous earth orbits (xe2x80x9cGEOxe2x80x9d) satellites have typically provided two types of services: (a) a relay mode, in which the GEO satellite merely relays a signal from one earth terminal to another, and (b) a broadcast mode, in which the GEO satellite transmits a signal to a large number of ground terminals. In the relay mode, also known as a xe2x80x9cbent-pipexe2x80x9d mode, a ground terminal transmits a signal using an uplink frequency to the GEO satellite, which retransmits the signal to a second ground terminal using a downlink frequency. This mode is illustrated in FIG. 1. Because the transmission footprint of the GEO satellite on the earth surface is large, the power density of the signal is very low. This requires that the receiving antenna be sufficiently large, ranging from one to three meters in diameter, to achieve the requisite antenna gain. However, these large antennas are practical only for large, commercial users. Individual consumers cannot afford the space or expense of these large ground antennas. Individual consumers are willing to tolerate only small antennas, such as those used for direct broadcast satellite (DBS) transmissions, which are typically one to two feet in diameter.
Small ground antennas often operate with a xe2x80x9chubxe2x80x9d service, in which the user uplink is routed from the satellite to an intermediate ground station known as the xe2x80x9chub.xe2x80x9d This service is illustrated in FIG. 2. The hub usually acts a local control center to assign channels and other functions associated with network management. This intermediate xe2x80x9cstopxe2x80x9d adds an additional xc2xc second to the propagation delay associated with the round trip to synchronous orbit, so the total delay in one terminal transmitting to another is approximately xc2xd secondxe2x80x94a delay many consider too long for viable high quality telephony today. The GEO satellite can also operate in a xe2x80x9cmeshxe2x80x9d configuration in which the user downlink is routed directly to the other user without the hub transmission.
In the broadcast mode, a hub or xe2x80x9cfeeder linkxe2x80x9d sends the entire spectrum of broadcast signals to the GEO satellite, which then rebroadcasts the signals to the region of interest. It is important to note that in the broadcast service all users receive the same signals, which are typically transmitted at nearly equal power levels because the ground terminals are assumed to receive the entire band of signals everywhere. The broadcast spectrum is divided up into a number of transponder bandwidths, each of which can carry a multiple of standard TV channels, high definition TV, or data. This type of transmission has become especially important in the direct broadcast satellite (xe2x80x9cDBSxe2x80x9d) of standard broadcast television as a competing service to cable.
Typically, GEO SATCOM systems use a single wide area coverage beam with a diameter of approximately 2,500 miles to provide complete coverage of CONUS. Therefore, in order for a ground antenna to receive adequate signal strength, the transmitter on the satellite must have sufficient power to provide an adequate power density within the single wide area coverage beam. However, this greatly increases the cost and complexity of the GEO satellite.
Another way to ensure that the ground terminal receives adequate signal strength is to use a ground station with a large diameter antenna to achieve the requisite gain. However, as the size of the antenna increases, so does the expense. Therefore, only commercial users are able to afford these antennas. Clearly, this solution is unacceptable to individual users, who demand cheaper, more aesthetically pleasing, smaller antennas.
Several attempts have been made to address this problem. One solution is to use a number of smaller spot beams instead of a single wide area coverage beam to cover the same geographical area. By decreasing the size of the spot beams while maintaining the same overall transmitted power, the power density within each spot beam increases. The increase in the power density within each spot beam enables the use of smaller ground antennas.
However, conventional systems employing spot beams typically only employ a single hub for the entire system. For example, in Europe, SES is preparing to deploy the ARCS system using the Astra 1H and 1K satellites to provide multi-beam coverage of Europe. The Astra 1H uses a standard Ku-Band direct video broadcast (xe2x80x9cDVBxe2x80x9d) downlink and a xe2x80x9cpiggybackxe2x80x9d Ka-band payload which routes individual user Ka-band uplinks to a single, central hub located in Luxembourg. The ARCS system uses eight beams on the Ka-band uplink, with each beam having a footprint of approximately 500 miles diameter on the earth to provide complete coverage of Europe. As a result of this high gain from the satellite receive antenna, a ground antenna of only 75 cm diameter with a xc2xdW transmitter can provide a 144 Kbps return channel data rate. The Ka-band uplinks from all eight of the beams are returned to the single hub in Luxembourg. The Ku-band data for the DVB is broadcast up from a feeder link to the satellite from the Luxembourg hub and is broadcast down to the area covered by all eight spot beams through a single broadcast beam with Ku-band DVB transponders. A single-hub ARCS system employing spot beams is illustrated schematically in FIG. 3.
Other satellite systems being planned now propose to provide a bent-pipe mode between individual ground terminals in different spot beams. These satellites plan to use digital processing on board in order to route the signal from one spot beam to another, which greatly increases the cost of the system.
Thus, there is a general need in the art for a SATCOM system using multiple spot beams to cover at least selected areas of the CONUS. There is a further need in the art for a SATCOM system which has a hub in every spot beam.
The present invention meets the above-described need by providing a SATCOM system having ground terminals, hubs, and at least one satellite stationed in a geosynchronous earth orbit (GEO) about the earth. The GEO satellite generates a network of spot beams covering selected area(s). A single hub and at least one ground terminal reside within each spot beam. A user terminal with a well-defined protocol can transmit an uplink signal to the hub through the GEO satellite. The user terminal also can receive a signal having a second well-defined protocol through the downlink spot beam from the hub through the GEO satellite. For example, the uplink from the ground terminal might use a MF/TDMA multiple access method to maximize the number of users who can be xe2x80x9con-linexe2x80x9d at given time. The corresponding downlink signal might may use the standard xe2x80x9cDVB-Sxe2x80x9d protocol, which supports both video and data transmissions.
The invention may also support a mode of operation where several individual spot beams shall share a single hub in a xe2x80x9cparent/dependentxe2x80x9d operational mode. Through selective frequency and/or polarization routing on board the satellite, a hub located within a xe2x80x9cparentxe2x80x9d beam would communicate with user terminals within the parent beam at a specified frequency and polarization and would communicate with users in other xe2x80x9cdependentxe2x80x9d beams on a different frequency and/or polarization. This routing would divide the total available bandwidth between these parent and dependent beams. Routing on board the satellite could be implemented to allow eventual separation between the parent and dependent beam by inclusion of a switch built into the on-board routing. This would allow full use of the available bandwidth in each beam. This method of deployment could allow a more gradual installation of hubs to restrict ground equipment costs at the beginning of service provision.
The invention may also support a second class of service, in which the hub downlink uses a second protocol that is adopted for transmission from a xe2x80x9ccommercialxe2x80x9d ground terminal. The commercial ground terminal may use this second protocol for both the uplink and downlink signals to facilitate the transmission of data at high speed from a remote site. This type of terminal can play the role of the hub in terms of transmitting directly on the downlink to xe2x80x9cresidentialxe2x80x9d terminals in either a local spot beam mode or a broadcast mode to all spots beams simultaneously.
The invention may also include xe2x80x9cintra-Beamxe2x80x9d and xe2x80x9cinter-Beamxe2x80x9d services, in which the capacity of the system is optimized by a coordinated network operation control center (xe2x80x9cNOCCxe2x80x9d). The NOCC can assign uplink frequencies and polarizations to individual ground terminals based on the signal destination, for both intra-beam and inter-beam transmissions. The NOCC may also assign a frequency bandwidth compatible with a narrow-band uplink (the residential service) or a wide-band uplink (the commercial service). Protocols for the residential and commercial match the two protocols used by the hub. A portion of the uplink band is assigned to each service. The NOCC may also allocate a frequency band and polarization to designate the type of service based on whether the communications link is inter-beam or intra-beam.
The invention may also include a router for directing the signal to the appropriate spot beams for inter-beam transmission. The router may operate in one of two modes. First, the router may direct the signal to the appropriate spot beam by selection of the frequency used for the uplink. Alternatively, the router may also direct the signal to the appropriate spot beam based on the signal polarization. Additionally, the router may also be used with the broadcast mode. For the broadcast mode, the selection of a particular frequency sub-band and/or polarization routes the uplink signal to into every downlink beam. Alternatively, the sub-band and/or polarization may be routed to the NOCC for a xe2x80x9cdouble-hopxe2x80x9d rebroadcast to all downlink beams.
The invention may further provide for power control in each downlink spot beam to optimize system capacity and throughput. Additionally, due to the individual spot beams being small and covering a localized area, the local weather conditions and geographical location can be factored into to the power control for each beam. Additionally, by utilizing power control, channel allocation can be optimized to allow greater numbers of channels in spot beams that encompass heavy population centers and fewer channels in spot beams covering less densely populated areas.
The invention may also provide a high-speed wide area network to connect each hub in each spot to every other hub. The high-speed WAN may be high-speed optical fiber, conventional landline connections, satellite links, or the like.